Threshold switches are devices which change their electrical conductivity in response to a voltage applied thereacross. Ovonic threshold switches are solid state devices based upon materials and effects first discovered by S. R. Ovshinsky; see for example, "Reversible Electrical Switching Phenomena in Disordered Structures" Physical Review Letters, vol. 21, no. 20, Nov. 11, 1968, p. 1450(c). Ovonic threshold switching materials have bi-stable conductivity characteristics. In the absence of an applied threshold voltage, the materials are in a blocking state and exhibit a high electrical resistivity. Imposition of a voltage exceeding a specific threshold value causes the materials to switch to a low resistivity conductivity state, which is many orders of magnitude below the resistivity in the blocking state, and this state is maintained provided a minimum holding current flows therethrough; termination of the holding current cycles the material back to the high resistivity state. Ovonic threshold switching devices exhibit switching speeds of less than 150 pico seconds and have significant use as high-speed transient suppressors as well as in various other switching applications. The characteristics of these switches and the materials from which they are fabricated are disclosed in U.S. Pat. Nos. 3,271,591 and 3,343,034, the disclosures of which are incorporated herein by reference.
Threshold switching devices are generally constructed to include a body of switching material with at least a pair of electrodes operatively disposed on opposite sides thereof. Control of the characteristics of the switching material is achieved by compositional modification and/or modifications of the local order or other morphological modification of the switching material. The switching material is designed to have a threshold value and an electrical resistance suited for a particular application.
"First Fire Voltage" is the voltage which is needed to switch a freshly prepared threshold switching device from the high resistivity to the low resistivity state. This first fire voltage is typically much higher than the normal threshold voltage manifested by the switch after first fire. The initial voltage permits morphological relaxation of the material to occur and thereby restructures the local order of the material to a stable configuration. Once the forming operation has been carried out, the threshold switching voltage remains reproducible and constant.
In the case where discrete switches are being manufactured, the first fire voltage phenomenon generally presents at most only a minor problem since, in an initial stage in the testing of the devices, they are simply exposed to a high voltage pulse and consequently formed. However, it is often desirable to include threshold switches in array configurations or in combinations with other devices and under such circumstances, the first fire voltage phenomenon can present a significant problem. The higher voltage necessary to form the freshly manufactured threshold switches may be sufficiently large so as to damage other semiconductor devices connected thereto. Also, if an array of threshold switches is being first fired, the first switch in the array to achieve its working threshold will then serve to create a low resistivity current path which prevents the other switches from being properly formed. To overcome these problems, sophisticated isolation circuitry must be included in combination with the threshold switches. Another problem presented by the first fire phenomenon is tied into the fact that the final working threshold voltage of the switch is, to some degree, a function of the first fire voltage applied to the device and the first fire voltage required may vary from device to device. Consequently, individual switches in arrays of threshold switching devices may all manifest somewhat different working thresholds.
Clearly, problems occasioned by the first fire phenomenon cam complicate the use of threshold switching devices in cross-point switching arrays and the like and in conjunction with other types of devices. Therefore, it would be desirable to provide threshold switching devices which are free of the effects of this phenomenon. Heretofore, it has been known to treat chalcogenide threshold switching materials with activated hydrogen for purposes of stabilizing the working threshold of such devices; these techniques are disclosed in U.S. Pat. No. 4,804,490, the disclosure of which is incorporated herein by reference.
In accord with the principles of the present invention, it is now possible to provide threshold switches which include fluorinated switching materials having optimized short range order and which manifest very little, if any, first fire voltage effects. The present invention makes possible the widespread utilization of threshold switches in a variety of circuits and in a variety of electronic devices and makes possible the manufacture of switching arrays having uniform properties. These and other features and advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.